ABSTRACT Key Words: Electronic fuel injection, Microcontroller, CNG, Manifold injection. Manifold injection with uniflow scavenging. Manifold injection with uniflow stratified scavenging. Direct CNG injection. Two-stroke engines have various applications in automobiles, sawmachines on the level surface, motorboats in river and seawater, unmanned air vehicles in space etc.. Such a vast range of application gives an ample scope for applying the modular concept in the design of two stroke engines. Two stroke spark ignition engines have a number of potential advantages over four stroke engines such as, high specific power output, simple construction, reduced part load pumping losses, compactness, lower number of mechanical components, lower production and maintenance costs. However, it has the inherent drawbacks of poor scavenging and relatively high exhaust emissions. This is mainly due to the short-circuiting losses of the fresh air fuel mixture into the exhaust and irregular combustion that occurs due to exhaust gas dilution. Over the years, various methods have been suggested to improve power output and reduce the exhaust emissions from a two-stroke engine. Coating of piston and combustion chamber using non-noble metals, use of catalytic converter, exhaust gas re-circulation, charge stratification, fuel injection system etc. Elimination of short-circuiting losses will improve the specific fuel consumption and reduction of exhaust emissions substantially. To eliminate the short-circuiting losses, direct injection is the best solution. In 111
this process, the fuel is injected directly into the cylinder after the closure of the exhaust port. Various methods such as cylinder head injection, transfer port injection, manifold injection, and cylinder bore injection have been tried using gasoline as fuel, with mechanical and electronic fuel injection systems. With the mechanical injection system, precise control of the injection timing and high-speed operation are difficult to achieve for gaseous fuel due to the mechanical inertia. Electronic fuel injection system is therefore an alternative to mechanical fuel injection system. The increased use of petroleum fuels by automobiles in India has not only caused fuel scarcities, price hikes, higher import bills, and economic imbalance but also causes health hazards due to its toxic emissions. Conventional fuels used in automobiles emit toxic pollutants, which cause asthma, chronic cough, skin degradation, breathlessness, eye and throat problems, and even cancer. In recent years, environmental improvement (CO2, NOx and Ozone reduction) and energy issues have become more and more important in worldwide concerns. Natural gas is a good alternative fuel to improve these problems because of its abundant availability and clean burning characteristics. In the present work elimination of short-circuiting losses was the main objective. This is achieved by IV
> Developing a manifold injection with uniflow stratified scavenging engine. > Developing a direct injection system. In this system the short-circuiting losses of the two-stroke engine is eliminated by directly injecting the fuel into the cylinder after closure of the exhaust port. The performance characteristics of an engine and the concentration level of the exhaust emissions depend, to a large extent, on the combustion pattern. It directly depends on fuel system, which provides an appropriate mixture of fuel and air to the engine at the appropriate point in the cycle. The fuel air mixture must be in right proportion as per the condition of the speed and load on the engine. The overall engine behavior depends upon the fuel induction mechanism. Introduction of a CNG kit to the existing gasoline engine hardware does not involve any substantial modifications except inducting the mixture into the intake manifold. However, in spite of the excellent characteristics and various advantages of CNG as a fuel in automobiles, it has certain problems, like backfiring during suction, knocking at higher compression ratio with advanced spark timing. These problems are due to inappropriate technology used for the formation of the air fuel mixture. In consideration of the inherent constraints in the design of carburetor, the engine manufacturers and automobile industries now are switching over to fuel injection system. By developing equivalent injection systems for a gaseous fuel, advance fuel control technologies of the gasoline vehicle can be implemented in the CNG vehicle. However, the flow rate of the natural gas injection is more than that of the gasoline injection.
The manifold injection system of four-stroke engine cannot be directly used in a two-stroke engine, because in four-stroke engine the fuel is injected once in every two revolutions of the crankshaft. In a two-stroke engine the fuel has to be injected once in every rotation of the crankshaft. Hence, highspeed injectors are required for two-stroke engine application. Another problem with the two-stroke engine is the fuel required per cycle, is lower compared to the four-stroke engine. In the present work, a microcontroller based gaseous fuel injection was developed for a two-stroke spark ignition engine. The present work has three phases. The first phase is the development of microcontroller based fuel injection system. The fuel was injected into the manifold to simulate the performance of the carbureted engine. Further the injection was optimized for better performance and minimum emissions. The engine with the manifold injection system was able to operate with relatively lean air-fuel mixtures and hence fuel consumption and the exhaust emissions decreased. The second phase of the work was the Development of the manifold injection with uniflow stratified scavenging. An uniflow two-stroke engine assisted by crankcase supercharging was chosen to provide the best trapping efficiency. With this type of engine it is possible to decrease the characteristics short-circuit of loop-scavenged engines. It is then possible to obtain the best fuel economy and power. VI
For further reduction in the loss of fresh charge through the exhaust valve, stratified scavenging was developed. In this system reed valves were fitted in the transfer duct. Fresh air through the reed valve enters into the scavenging ports before the entry of fresh charge into the cylinder through transfer ports. This helps in reducing the loss of fresh charge through the exhaust valve. The emissions and specific fuel consumption further decreased due to improved scavenging. Manifold injection(mi), and manifold injection with uniflow stratified scavenging(miufss), although improved the efficiency and reduction of emissions, the short-circuiting losses were not completely eliminated. In the third phase direct injection system was tried. The third phase of the work was the development of direct injection system. The short-circuiting losses of the two-stroke engine can be eliminated by directly injecting the fuel into the cylinder after the closure of the exhaust port. In the direct injection system, injection nozzle was inclined at 40 from the horizontal and is located in the cylinder wall. This injects the fuel upward. The injection angle was optimized for the best engine performance. It was found that the direct injection system had the best performance and low emissions due to elimination of short-circuiting losses. Various systems tried in the order of effectiveness can be ranked as: > Direct injection system > Manifold injection with uniflow stratified scavenging > Manifold injection All these systems are superior to the carbureted version. Vll